Mouth whistles have been used for some time for musical as well as signal purposes.
U.S. Pat. No. 733,122, issued Jul. 7, 1903, describes a musical type mouth whistle with a horn section in front and a laterally straight, folded over, rear wall. The horn section has upper and lower walls with laterally extending apertures for engagement with the lips of the player to prevent ejection of the whistle.
U.S. Pat. No. 805,817, issued Nov. 28, 1905, also describes a whistle with an anterior horn section with ridges to assist retention in the mouth of the user and with a reed within the horn. This whistle also has a laterally straight rear wall.
British patent 241,974, granted Nov. 5, 1925, shows a mouth-held whistle formed of a metal blank of sheet metal folded upon itself, with substantially opposed holes in the upper and lower walls adjacent the rear wall, the rear wall being formed by simply bending a circular blank upon itself along substantially the diametric line. The whistle is provided with an edge closure or sidewall adjacent and connected to the lateral ends of the sidewall to form an amplifying chamber, or, the opposed edges of the upper and lower walls are ground or cut away adjacent the rear wall in an arcuate shape to receive a thumb and forefinger, respectively, of the user, to provide the desired side closures. The rear wall of this whistle is also defined along the straight line of the linear fold of the metal blank.
Australian patent 136,866, granted Apr. 3, 1950, shows and describes another whistle made by bending a circular or octagonal blank of sheet metal upon itself, but also by forming the opposed complementary metal walls adjacent the fold into a bulbous chamber to allegedly improve the tonal qualities of the note produced. By utilizing a springy sheet metal, the user can exercise further control of the emitted sound by pressing the upper and lower walls closer together or releasing the walls as desired. As in the other whistles described, the whistle has a liearly straight rear wall that is inserted into the mouth of the user.